1. Field of Invention
The present application relates to methods and apparatuses for suppressing thermally induced motion of a workpiece.
2. Description of Related Art
Numerous applications involve thermally induced motion of a workpiece. For example, in the manufacture of semiconductor chips such as microprocessors, the workpiece typically includes a semiconductor wafer, supported in a thermal processing chamber for annealing or other heat-treating purposes. U.S. patent application Ser. No. 10/742,575 (publication no. US 2004/0178553 A1), discusses examples of heat-treating techniques for annealing such semiconductor wafers, in which the wafer is first pre-heated to an intermediate temperature, following which the top or device side surface is rapidly heated to an annealing temperature. The initial pre-heating stage occurs at a rate significantly slower than a thermal conduction time through the wafer, and may be achieved by irradiating the back-side or substrate side of the wafer with an arc lamp, to heat the wafer at a ramp rate less than 400° C. per second, for example. The subsequent surface heating stage occurs much more rapidly than the thermal conduction time through the wafer, so that only the device side surface is heated to the final annealing temperature, while the bulk of the wafer remains close to the intermediate temperature. Such surface heating may be achieved by exposing the device-side surface to a high-power irradiance flash from a flash-lamp or bank of flash lamps, the flash having a relatively short duration, such as one millisecond, for example. The cooler bulk of the wafer then acts as a heat sink to facilitate rapid cooling of the device side surface.
Such annealing methods, which involve rapidly heating the device side of the wafer to a substantially higher temperature than the bulk of the wafer, cause the device side to thermally expand at a greater rate than the rest of the wafer. The present inventors and the inventors of the above-noted U.S. applications have appreciated that, depending on the magnitude of the temperature difference between the device side temperature and the temperature of the bulk of the wafer, this may tend to cause “thermal bowing”, whereby the normally-planar wafer deforms itself into a thermally deformed shape. Depending on the magnitude and rapidity of the device side heating stage, the thermally deformed shape may have attributes of a dome shape, with the center of the wafer tending to rapidly rise relative to its edge regions. The thermal bowing may also cause the outer perimeter or edge of the workpiece (such as the outer one or two centimeters of a 30-cm wafer, for example) to curl downward steeply, and thus, the thermally deformed shape may also have attributes of a FRISBEE(™) flying disc. In practice, for some applications it has been found that the latter curling effect at the outer perimeter of the workpiece tends to be more pronounced than the former dome-shaped curvature of the workpiece as a whole, although this may depend on the physical parameters of the thermal cycle in question. The thermally deformed shape represents a minimum stress configuration of the wafer, minimizing the thermal stress resulting from the temperature gradient between the device side and the bulk of the wafer. However, due to the extreme rapidity at which the device side of the wafer is heated (in the course of a 1-millisecond flash, for example, much faster than a typical thermal conduction time in the wafer), the deformation of the wafer may occur sufficiently rapidly that the edges of the wafer apply a large downward force onto the support pins that support the wafer in the chamber. As conventional support pins are typically rigid, the resulting reaction force between the pins and the edges of the wafer may damage the wafer. Such forces may also cause the wafer to launch itself vertically upward from the support pins, which may result in further damage to the wafer as the wafer falls back down and strikes the pins. Also, as conventional support pins are not designed to withstand such forces, they tend to break, with the result that the wafer falls in the chamber and is damaged or destroyed. In addition, due to the rapidity at which such thermal bowing occurs, the initial velocities imparted to the various regions of the wafer tend to cause the wafer to overshoot the equilibrium minimum stress shape and oscillate or vibrate, resulting in additional stress and potential damage to the wafer.
The above-noted U.S. application publication No. US 2004/0178553 A1 discloses numerous approaches to overcoming such difficulties, by supporting the workpiece in a manner which allows the workpiece to move under the influence of the resulting thermal stresses in order to minimize its own internal stress, thereby reducing the likelihood of wafer damage or breakage. Approaches to suppressing vibration or oscillation of the wafer are also disclosed, however, further suppression would be desirable, as such vibrations may tend to damage or break the wafer, depending on the surrounding circumstances of the particular annealing method in question.
A recently-proposed variant of the previously-mentioned annealing method involves substituting a hot plate for the arc lamp during the pre-heating stage. The subsequent device-side surface heating stage employs a bank of flash-lamps to produce a high-power irradiance flash. The wafer is held in tight contact with the hot plate by a vacuum chuck, which includes gas channels or passageways defined through the hot plate, and a compressor to pump gas out of the passageways, to create a vacuum in the passageways, immediately beneath the back-side of the wafer. The vacuum in the channels effectively sucks the wafer tightly against the hot plate surface, during both the pre-heating and subsequent device-side surface heating stage. However, the vacuum chuck prevents the wafer from thermally bowing during the device-side surface heating stage, thereby preventing the wafer from deforming itself in order to minimize its own internal stress. As a result, the wafer tends to break, particularly if the magnitude of the “jump” (i.e., the difference between the intermediate temperature and the ultimate device-side temperature) is large. Thus, such constrictive approaches to suppressing motion or vibration of the wafer are undesirable.
Accordingly, there is a need for an improved way of suppressing thermally induced motion of a workpiece.